1. Field of the Invention
The present invention relates to initiators comprising ignition charges and to a method for assembling such initiators.
2. Related Art
U.S. Pat. No. 4,727,808, issued Mar. 1, 1988, to Wang et al, discloses an electrically-initiated detonator, an igniting means such as fuse head (9) or an electric resistance wire, low energy detonating cord, NONEL tube or safety fuse (see column 4, lines 41-44 and column 7, lines 21-28) and an initiating charge in initiation relation thereto. The initiating charge comprises a secondary explosive, such as PETN (pentaerythritol tetranitrate), RDX (cyclo-1,3,5-trimethylene-2,4,6-trinitramine), or a mixture thereof, with a particle size that may be below 30 micrometers (xcexcm) and which may be pressed to a density in the range of 1.2 to 1.6 grams per cubic centimeter (g/cc) (see column 5, lines 11-32). The initiating charge is used to initiate the base charge of the detonator. An intermediate charge may be disposed between the initiating charge and the base charge and may have an even lower density, e.g., to 0.8 to 1.4 g/cc (see column 5, lines 33-45). Example 7 shows a test employing PETN at 5 to 15 xcexcm particle size and a tamping of 133 kg (about 8660 psi) for a containment shell having an outer diameter of 6.5 millimeters (mm) and a wall thickness of 0.6 mm.
The xe2x80x9cigniting meansxe2x80x9d mentioned in the Wang et al Patent draw or emit large amounts of energy relative to low energy initiation elements such as SCBs. Further, given the types of igniting means contemplated by Wang et al, the function time for the detonators disclosed therein will be on the order of about 50 microseconds. Because of this prolonged function time, the Wang et al detonators need to provide the confinement and empty chamber in the detonator to prevent the detonator shell from being destroyed by the gaseous products of the ignition charge before the detonation reaction is initiated in the base charge. In the embodiment of FIG. 13, the hollow interior of safety fuse 16 provides the empty chamber for this device.
Fyfe et al, in a paper entitled xe2x80x9cBNCP Prototype Detonator Studies Using a Semiconductor Bridge Initiatorxe2x80x9d, discloses the use of BNCP (tetraammine-cis-bis (5-nitro-2H-tetrazorato-N2) cobalt (III) perchlorate) for use in electric detonators incorporating a semiconductor bridge (SCB) in welded 304 stainless steel confinements. One test device comprised 25 milligrams of BNCP pressed to 10,000 pounds per square inch (psi); another comprised 49 milligrams of BNCP pressed to 20,000 psi. Ignition sensitivity tests for two different particle sizes of BNCP, 15 and 25 microns, performed with a rise time of 15 microseconds, showed that the larger particles took about twice as long to ignite as the smaller particles at 3.5 amps and, at 1.5 amps, the smaller particles ignited but the larger particles did not. In addition, at a fifty-microsecond rise time, the smaller particles were less temperature-sensitive than the larger particles.
The SCB employed by Fyfe et al measured 90xc3x97270xc3x972 xcexcm, and consumed several millijoules of energy to ignite the BNCP. The reported 1 watt, 1 ampere no-fire of these detonators indicates that the BNCP charge was acting like a heat sink that quickly dissipated the ohmic heating of the SCB at the 1 watt, 1 amp no-fire current. Such heat absorption under no-fire conditions indicates that the BNCP was highly compacted.
A manufacturer of BNCP has published product literature suggesting the use of BNCP in place of lead azide as a primary explosive initiating charge and that BNCP is a DDT explosive with a theoretical maximum density of 2.03 g/cc.
U.S. Pat. No. 4,484,960 to Rucker, dated Nov. 27, 1984, discloses a bridgewire detonator comprising a boron/ferric oxide ignition composition. The ferric oxide particles are in the 0.2 to 1.2 xcexcm range. In the example, the ignition composition is loosely loaded into a blasting cap shell in contact with the bridgewire.
U.S. Pat. No. 4,989,515 to Kelly et al, dated Feb. 5, 1991, discloses an iriter comprising a bridgewire in contact with an ignition charge comprising thermite, an incendiary composition. The ignition charge is in contact with a thermite output charge. The ignition charge is compacted to 50-70% of its theoretical maximum density (TMD) while the output charge is compacted to 90-99% TMD.
In one broad aspect, the present invention relates to an initiator such as a detonator or a pyrotechnical output initiator that comprises a specifically configured ignition charge. Thus, the invention provides an initiator comprising a housing, a low-energy electronic initiation means in the housing, and an ignition charge disposed in the housing in direct initiation relation to the initiation means and in a state of compaction of less than 7000 psi. The ignition charge serves to produce a deflagration signal in the housing in response to a low-energy initiation signal from the initiation means, and it comprises particles having an average particle size of less than 10 xcexcm. There is also an output charge in the housing for producing an output signal in response to the deflagration signal of the ignition charge.
According to one aspect of the invention, the ignition charge may be disposed in a pulverulent form and may be subjected to a compaction force of less than 5880 psi. For example, the ignition charge is subjected to a compaction force of less than 3000 psi, or less than 2000 psi.
Preferably, the ignition charge comprises BNCP.
In accordance with another broad aspect of this invention, there is an initiator comprising an initiation means for producing an initiation signal that releases less than about 850 microjoules into the housing. Optionally, the initiation means may release less than about 425 microjoules into the housing, or less than about 250 microjoules, or even less than about 100 microjoules into the housing.
It is generally preferred that the ignition charge comprise BNCP particles having an average size of less than 10 xcexcm, or less than 5 xcexcm, e.g., having an average diameter in the range of from about 0.5 xcexcm to 2 xcexcm.
Typically, the initiation means comprises a semiconductor bridge (SCB) initiation element.
According to still another broad aspect of this invention, the initiator comprises an ignition charge disposed in a state of compaction of less than 65.9 percent of its theoretical maximum density (TMD). For example, the ignition charge may be disposed in a pulverulent form and is in a state of compaction in the range of from about 49 to 65 percent of its TMD, or in the range of from about 49 to about 59 percent of its TMD.
In more specific embodiments, the invention provides a low-energy initiation unit in the housing comprising an SCB and an ignition charge disposed in the housing in direct initiation relation to the SCB. The ignition charge may comprise BNCP having a particle size of less than 10 xcexcm average diameter and in a state of compaction of less than 7000 psi.
Optionally, the ignition charge may comprise an adherent bead disposed on the SCB. The bead may comprise a mixture of BNCP and a binder.
In a particular embodiment, the initiator may comprise a containment shell secured to the initiation means in the housing, and the ignition charge may be disposed within the containment shell.
The invention also encompasses a method aspect, e.g., a method of assembling an initiator. One such method comprises pressing an output charge into a housing, disposing a pulverulent ignition charge into the housing in signal transfer relation to the output charge, securing an electronic initiation means in the housing in initiation relation with the ignition charge, and compacting the ignition charge with a force of less than about 5880 psi.
In another embodiment, the method may comprise pressing an electronic initiation means into an ignition charge with a force of less than about 5880 psi, securing the ignition charge to the initiation means, and then securing the ignition charge in the housing in signal transfer relation with the output charge, preferably without farther compacting the ignition charge.
In yet another embodiment, the method may comprise depositing a bead of ignition charge on an electronic initiation means, and securing the electronic initiation means in the housing with the ignition charge in initiation relation with the output charge in the housing.